Rails 4 访问连接表属性

Question

我有一个 has_many through 用于配方应用程序的连接表设置，其中 Ingredient 和 Meal 通过 MealIngredient 连接。在MealIngredient 中，我有meal_id、ingredient_id 和amount。我的问题是：如何访问amount 列？

在我的食谱视图中，我循环浏览配料：

@meal.ingredients.each do |i|

我可以从MealIngredient 加入记录中访问成分的属性，但不能访问数量。

我尝试在查询中使用includes 执行@meal.ingredients.includes(:meal_ingredients)，但我不确定如何在上述循环中访问amount。当我使用i.inspect 时，我根本看不到对meal_ingredients 表的任何引用。

有没有办法使用i.amount访问该循环中的变量？

提前感谢您的帮助！

Answer 1

于 2020 年 8 月 1 日更新 - RPECK

为此苦苦挣扎了几个月，直到我们找到了合适的解决方案：

--

ActiveRecord Association Extensions

您遇到的问题是 Rails 只会使用连接表中的 foreign_keys 来加载您需要的关联数据。除非您实际直接加载连接模型，否则它不会让您能够访问连接属性

一些搜索将我们引向ActiveRecord Association Extensions - 一种访问不同 ActiveRecord 关联之间的中间数据的方法（使用称为 proxy_association 的集合）。这将允许您从连接模型访问额外的属性，将它们附加到您的“原始”模型：

#app/models/ingredient.rb
class Ingredient < ActiveRecord::Base
   attr_accessor :amount #-> need a setter/getter
end

#app/models/meal.rb
class Meal < ActiveRecord::Base
   has_many :meal_ingredients
   has_many :ingredients, { -> extending: IngredientAmount }, through: :meal_ingredients
end

#app/models/concerns/ingerdient_amount.rb
module IngredientAmount

    # => Load
    # => Triggered whenever module is invoked (allows us to populate response data)
    # => #<Method: ActiveRecord::Relation#load(&block) c:/Dev/Apps/pwinty-integration/.bundle/ruby/2.7.0/gems/activerecord-6.0.2.1/lib/active_record/relation.rb:614>
    def load(&block)

       # => This is called from the following reference - if you want to dig deeper, you can use method(:exec_queries).source_location
       # => c:/Dev/Apps/pwinty-integration/.bundle/ruby/2.7.0/gems/activerecord-6.0.2.1/lib/active_record/association_relation.rb:42
      unless loaded?
        exec_queries do |record|
          record.assign_attributes({amount: items[record.id]}) if items[record.id].present?
        end
      end

    end

    # Load
    # Deprecated with AR 6.0
    #def load
    #   amounts.each do |amount|
    #       proxy_association.target << amount
    #   end
    #end

    #Private
    private

    #Amounts
    # Not needed after AR 6.0
    #def amounts
    #   return_array = []
    #   through_collection.each_with_index do |through,i|
    #       associate = through.send(reflection_name)
    #       associate.assign_attributes({amount: items[i]}) if items[i].present?
    #       return_array.concat Array.new(1).fill( associate )
    #   end
    #   return_array
    #end

    #######################
    #      Variables      #
    #######################

    #Association
    def reflection_name
        proxy_association.source_reflection.name
    end

    #Foreign Key
    def through_source_key
        proxy_association.reflection.source_reflection.foreign_key
    end

    #Primary Key
    def through_primary_key
         proxy_association.reflection.through_reflection.active_record_primary_key
    end

    #Through Name
    def through_name
        proxy_association.reflection.through_reflection.name
    end

    #Through
    def through_collection
        proxy_association.owner.send through_name
    end

    #Captions
    def items
        #through_collection.map(&:amount)
        through_collection.pluck(through_source_key, :amount).map{ |id, amount| { id => amount } }.inject(:merge) #-> { id: record, id: record }
    end

    #Target
    # This no longer works with AR 6.0+
    #def target_collection
    #   #load_target
    #   proxy_association.target
    #end

end

这应该现在将amount 属性附加到您的ingredient 对象，允许您执行：

@meal = Meal.find 1
@meal.ingredients.each do |ingredient|
   ingredient.amount
end

Answer 2

在这种情况下，您应该循环访问meal_ingredients 关联。您应该立即加载 ingredients 关联以减少数据库查询。

@meal.meal_ingredients.includes(:ingredient).each do |meal_ingredient|
  puts meal_ingredient.amount
  puts meal_ingredient.ingredient.name
end

更新

此更新是在 Rich Peck 的回答之后发布的，但我认为有一种更简单的方法可以实现他所做的。

@meal.ingredients.select('ingredients.*, meal_ingredients.amount').each do |ingredient|
  puts ingredient.amount
  puts ingredient.name
end